Controls on free-living nitrogen fixation during decomposition of leaf litter across a soil-age gradient

Background/Question/Methods

Free-living nitrogen (N) fixation is a vital ecosystem process providing a new source of N into ecosystems during litter decomposition; however, few studies have tested the controls on N fixation during decomposition. Here we tested the relative influence of site conditions versus litter substrate quality on N fixation during decomposition. We tested the hypothesis for home field advantage (HFA) during decomposition for both rates of mass loss and rates of litter N fixation, predicting higher rates to occur at a specific litter’s home environment. We tested this hypothesis across a soil-age gradient in Fort Bragg, CA. representing 100,000 to >1,000,000 years of soil development. The younger two terraces (1 and 2) support a productive mixed conifer forest, while the older terraces (3-5) support a pygmy forest of stunted vegetation. We established a litterbag decomposition experiment using senescent Bishop Pine (Pinus muricata) litter from terraces 1, 2, and 4.  Litter was decomposed in situ and a reciprocal experiment with a common litter source (terrace 2) and a common site of decomposition (terrace 2). We collected litterbags six times over the course of two years and at each time measured rates of N fixation, mass remaining, and litter chemistry. 

Results/Conclusions

We found evidence for the HFA hypothesis on rates of mass loss and rates of N fixation for only fertile litter (terrace 2) indicating strong context dependence for the HFA hypothesis. Within the reciprocal transplant experiment we found that the site of decomposition (i.e. decomposer community and nutrient availability) had a significant effect on rates of litter N fixation during the course of decomposition (p<0.005) while litter source (p=0.55) did not.  For example, low quality litter (terrace 4) had higher rates of N fixation when decomposed at the fertile site (terrace 2) versus when decomposed at the infertile site. We observed similar patterns for mass loss; the site of decomposition had a significant effect on mass loss (p<0.005) while litter source did not (p=0.16). Patterns in phosphorus immobilization during decomposition suggest this element is a strong regulator on free-living N fixation. In the reciprocal experiment, infertile litter when decomposed at the fertile site immobilized phosphorus until the litter in the bag matched the phosphorus content of the fertile litter.  Our study suggests that rates of free-living N fixation are influenced more by nutrient availability at a site than litter substrate quality.